1. Technical Field
The present invention relates generally to a system for pumping oil from a well, and more particularly to an artificial lifting system in which a composite tubing string is used for operating a reciprocating pump at the foot of the well and for transporting oil to the head of the well.
2. Background Art
Artificial lifting systems of the type commonly employed in the hydrocarbon production industry normally have a carbon steel tubing string which runs from a wellhead at the surface of the earth downwardly through a well bore to an oil bearing formation. The tube is connected at its lower end to a carbon steel or alloy pump, and the pump has a reciprocating plunger for extracting oil from the formation and transporting oil upwardly through the steel tube to the wellhead.
More particularly, the pump plunger is actuated in a reciprocating manner by a rod string which extends downwardly through the tubing string. The rod string typically is made of carbon steel or fiberglass, and is vertically lifted by a pump drive unit which is located at the wellhead and aligned directly over the well bore. Fluids which are extracted by the pump are ported into the annular space between the rod string and the tubing string. The fluids then are forced to the surface and discharged through a pumping tee to surface flow lines which, in turn, are connected to surface production and/or storage equipment.
Fluid containment at the wellhead is accomplished with a polished rod and stuffing box. The polished rod is connected to the upper end of the rod string, and is reciprocated through the stuffing box by the pump drive. The stuffing box contains a series of compression rubbers for accommodating movement of the polished rod without incurring fluid loss at the top of the tubing string.
A problem which exists with lifting systems of the character described is the likelihood of a leak developing in the tubing string, and further, the potential for structural failure in the rod string used to actuate the pump. Due to the abusive down-hole environments in which artificial lifting systems are used, and more specifically due to the deleterious nature of the fluids to which lifting systems are subjected, the tubing strings and rod strings of the current art are susceptible to corrosion, scaling and abrasive wear from fluid borne solids.
In addition, due to the minimal clearances between the rod strings and the interior surface of the tubing strings, abrasions causing tubing wear and rod failure are commonplace. As can be understood from the above discussion, either of these incidents results in a loss of operability in the lifting system and requires time-consuming and expensive repair.
The present invention is directed toward overcoming the problems set forth above and advancing the state of the lifting system art by utilizing the weight savings and corrosion resistance properties uniquely associated with composite materials. In addition, the invention is directed to various features employed with such a composite system.